Windows XP does not provide any method to display folder size in file-explorer, the only method to get the folder-size is to Right-click and select Properties, Folder Size is a nice little free utility to solve this problem, the utility adds a new "Folder Size" column to Windows Explorer displaying size of every folder.

Folder Size works in background calculating and caching the results for faster retrieval later, the service consumes around 5MB RAM and does not slow down your machine.